This invention relates to sol-gel related materials useful as carriers for optical identifiers in verification applications, to materials useful for coating or encapsulating, to novel, transparent non-linear optical materials and to ceramics.
A need exists for optically clear matrix materials which can suspend reflective particles for use as optical identifiers in verification applications, e.g., tagging of weapon components. Optical identifiers of this type must display unique optical signatures which can be used in identifying and verifying the authenticity of an object. Such applications typically demand a material with high optical transparency, extreme resistance to thermooxidative aging, good adherence to a variety of substrates, particularly metal oxides, a high degree of brittleness to thwart attempts to illicitly remove or transfer the tag, and sufficient mechanical stability to avoid cracking during thermal cycling, among other properties.
Sol-gels (materials formed by hydrolyzing orthosilicate esters, Si(OR).sub.4, with water under either acidic or basic conditions) substantially meet the above criteria except the last thermal cycling criterion; they tend to crack readily, causing problems such as providing false positives for tampering and delamination from the substrate. FIG. 1 is a simplified theoretical representation of a matrix 1 of polymer chains of such a sol-gel. The sol-gels bond well to substrates such as composite surfaces, metals and metal oxides. These characteristics would make sol-gels useful as tags for rocket engines, missiles and a variety of other applications, in the absence of the above identified problems.
Hybrid silicone and organic sol-gel-like materials are also known, e.g., as disclosed in Wilkes, et al., Silicon-Based Polymer Science: A Comprehensive Resource, Advances in Chemistry, Vol. 224, J. M. Zeigler and G. Fearon, Eds., American Chemical Society, Washington, D.C., 1990, p. 207 and in Schmidt, ACS Symp. Ser., 360, 333 (1988). FIG. 2 is a simplified theoretical representation of a matrix of a sol-gel-like structure 21 having rubbery polymers 22 bonded therein. These materials may be useful in tagging applications (i.e., as identifiers in verification applications), but they have an inherent disadvantage in that they must be formed into gels, loaded homogeneously with reflective particles to provide an optical signature, and applied on site. The application of such a gel in a uniform manner is a difficult, if not impossible, task, especially for non-technical field personnel carrying out a tagging operation which must result in a tag having substantially no cracks.
In a different aspect, sol-gel like materials might also be attractive in non-linear optical applications (e.g. waveguides, optoelectronics) if they had sufficiently high optical nonlinearities. Prior art sol-gel materials do not exhibit useful nonlinearities, but the many favorable properties of sol-gels for optical verification schemes also make the materials attractive for this type of use. The problem that must be solved for these materials to find application in this area is to provide a means of incorporating structures with a high optical nonlinearity in the sol-gel while preserving its useful optical, chemical, and physical properties. This has not been achieved to date in the prior art.
Sol-gel processed optical waveguides based on the .pi.-conjugated, carbon-based polymer poly-p-phenylenevinylene produced in situ in a tetramethylorthosilicate (TMOS) sol-gel have been prepared by Karasz and Prasad and mentioned in a recent review (D. R. Ulrich, Chemical and Engineering News, Jan. 1, 1990, pp. 28-40]. These composites are said to have "low" optical loss, stemming from absorption of the propagating light beam by the PPV part of the composite. PPV is known to have a strong absorption in the visible spectral regime, unlike the .sigma.-conjugated, silicon and/or germanium-based materials of this invention, which have no significant visible light absorption, even as pure polymers, and proportionately less when present in the sol-gel hybrid. In addition, .pi.-conjugated polymers like PPV have relatively poor thermooxidative stability and mechanical properties. The high stability and improved mechanical properties of the polysilanes and related materials of this invention, thus, is another advantage of the current materials over those of the prior art.